This invention relates to urea-aldehyde polymers and more particularly to a process for producing polymers, having properties of high structural strength and low residual aldehyde vapor, by the reaction of two storable solutions comprising partially cured aqueous urea-formaldehyde resin and aqueous dialdehyde cross-linking solution. The process for producing the new polymers is particularly useful in the manufacture of high strength, low odor, urea-aldehyde insulating foam, and high strength, low odor particle board and plywood.
The building industry consumes large amounts of urea-formaldehyde polymers in the manufacture of structural, decorative, and insulating boards, and in lightweight insulating foam. Urea-formaldehyde polymers have been particularly important because they may be formed and cured in relatively simple and economical procedures, and because they may be combined with other low cost materials, particularly cellulosic fibers and wood, to form composites which are economically useful building materials.
Urea-formaldehyde polymers developed to date have not had the structural strength required to produce an effective foam insulation or composite structural board, which would not generate objectionable and unhealthy amounts of formaldehyde vapor during the formation and service life of the products. Copending U.S. application Ser. No. 761,321, by this inventor, disclosed improvement in the chemical stability of urea-formaldehyde foam by reaction of 1-5% of dialdehydes, containing two to six molecular carbons, and additional urea, in the preparation of a partially cured resin for use in insulating foam manufacture. Although the said disclosure represented an advance in the art of producing insulating foams having properties of long term stability, it did not provide a process for the production of polymers with added structural strength required to provide load bearing properties or stiffness to building structural members. Some such structural members used for building include core fillings for thin-skin wall panels or door cores. More conventional structural uses include resin-filled chip or cellulose fiber panels. Further, the said disclosure did not provide a system in which the ingredient liquids are storable for practical periods of time nor a system which could be practically applied to the manufacture of composite boards and plywood. The partially cured resin containing 1 to 5 percent dialdehyde and added free urea must be used within a few days of its manufacture, to prevent the precipitation of an insoluble glyoxal-urea-urea-formaldehyde copolymer and deactivation of the partially cured resin.
Because of the realization of possible hazards from the continued evolution of formaldehyde from particle boards, plywood, and insulating foams, and interest by governmental regulatory agencies, there has been considerable evidence of progress in the art. Kawashima in Japanese Pat. No. 74 71,118 discloses the use of large amounts of calcium lignosulfonate to reduce free formaldehyde in luan plywood. The presence of large amounts of base-forming materials such as calcium, sodium, potassium, ammonia, or urea does reduce initial free formaldehyde content of the product but weakens rather than strengthens the final product. Dashkovskaya et al in USSR Pat. No. 480,555, Aug. 15, 1975 disclose that the addition of mineral oil containing sodium silicate reduced formaldehyde odor in particle board as produced. However, sodium silicate after heating and drying gives a basic reaction, so that repeated moistening and drying of the particle board in structural service will degrade and not strengthen the urea-formaldehyde polymer.
It is therefore a primary object of this invention to provide a process for producing from two storable liquids stable urea-aldehyde polymers having the structural strength required for use in structural members for the building industry.
It is another object of this invention to provide a process for producing urea-aldehyde polymers having high structural strength with the elimination of objectionable formaldehyde vapor emission during the polymer service life.
It is another object of this invention to provide a process for producing urea-aldehyde polymers from a partially cured urea-formaldehyde resin solution and a cross-linking solution, both said solutions having commercially acceptable storage life properties.
It is another object of this invention to provide a process for producing urea-aldehyde insulating foams having properties of increased structural strength and decreased formaldehyde vapor emission.
It is another object of this invention to provide a process for producing urea-aldehyde polymer-filled particle board having properties of increased structural strength and decreased formaldehyde vapor emission during the formation and service life of the board.
It is another object of this invention to provide a process for producing thin-skin foam filled building panels suitable for use in commercial construction applications.
These and other objects will be evident from the following description of the invention.